An alkylation unit at Citgo Petroleum Corp.'s Corpus Christi refinery was leaking for five months before spilling as much as 500 pounds of the hydrofluoric acid on March 5, the U.S. Chemical Safety Board said.

Leaks from a flange on the unit occurred as early as September, according to the statement from Johnnie Banks, lead investigator for the board. The release persisted after maintenance and the unit was kept online while a clamp was redesigned, he said.

Water cannons were automatically activated to contain the chemical release, Citgo said March 6.

"The facility is routinely using the water cannons as release mitigation for maintenance activities when in reality they should not be used as the last line of defense," Banks said in the statement.